1. Technical Field of the Invention
The present invention relates to methods and apparatus for displaying an eyeground three dimensionally and measuring its coordinates from photos of the eyeground.
2. Description of the Related Art
An eyeground means the inner portion of an eyeball, and is one of the few portions of a human body wherein blood vessels and nerves can be directly observed. Conventionally, dedicated eyeground cameras have been used to observe the eyeground and diagnose abnormalities in blood vessels, retinas, tumors, etc.
The eyeground camera illuminates the interior of an eye through a pupil of a patient, and at the same time the camera takes a picture of part of the eyeground on photo film or CCD. However, the field of view of the eyeground camera is narrow at about 50 degrees, so even a specialized physician cannot easily make a precise diagnosis using a photo.
Therefore, a number of eyeground photos are connected according to conventional practice. That is, because the imaging range of an eyeground camera is narrow, photos taken in a number of directions such as center, upper, lower, left, and right directions are connected together, and the entire eyeground is diagnosed. Such connecting work is done not only manually but also semi automatically using an image processing method. In addition, eyeground cameras connected to computers have been developed for electronic clinical cards and video images can be filed.
However, because the eyeground is spherical and a photo of it is a projection of part of the eyeground on a plane, photos taken are not continuous and cannot be precisely connected.
Eyeground photos connected as described above consist of planes developed from the spherical eyeground, so only a specialist physician can precisely diagnose such photos, applying extensive expertise of the construction of an eyeball, and a physician without specialized knowledge or a patient cannot understand the images.
Furthermore, even if a specialized physician can diagnose an abnormality in blood vessels, retinas, tumors, etc. at the eyeground, images on eyeground photos have already been deformed, therefore, sizes and positions of blood vessels, retina detachment areas, etc. can neither be quantified as data nor be used as time series data or as an epidemic database.
To solve the aforementioned problems, “Methods and apparatus for recovering three dimensional patterns of an eyeground by superimposing eyeground images taken in multiple directions” (unexamined Japanese patent publication No. 2002-34925) has already been proposed.
According to this proposal, the lens of a living eye is modeled with a single lens, wherein the shape of the eyeground is regarded to be substantially a sphere; then, an image of the sphere taken through the single lens lies on a quadratic surface, and it can be assumed that the camera has taken pictures of the quadratic surface from various directions. Consequently, as shown schematically in FIG. 1, a number of unique points commonly contained in a number of eyeground photos are superimposed on the quadratic surface, thus camera positions and poses, when the shape of the quadratic surface and pictures were taken and estimated, the parameters of a single lens are obtained from the shape of the quadratic surface, and the shape of the eyeground can be reproduced.
Nevertherless, actual sizes and shapes of eyeballs vary by person, and it is known that if a person who is very near or far sighted in particular, the eyeballs deform from a precise sphere. Therefore, if the lens of a living eye is modeled with a single lens and the shape of the eyeground is regarded to be substantially a sphere, in the case of a deformed eye ball, if any, the reproduced eyeground differs from the actual eyeground, so the fact that the reproduced eyeground cannot be used for precision diagnostics or a database of eyeground data, is a problem.
According to the above mentioned method, if an eye ball is deformed significantly, images cannot be superimposed on a virtual quadratic surface. As a result, camera positions and poses when each image is photographed can no longer be estimated, so parameters for a single lens also cannot be obtained, and even the shape of the eyeground might not be reproducible.